Hyperplaty clays and their use in paper coating and filling, methods for making same, and paper products having improved brightness

ABSTRACT

The present invention relates to a hydrous kaolin product having improved optical properties, for example, when used in the production of paper products. The present invention comprises an improved barrier coating for paper and a method of making the coated paper. The present invention also comprises an improved method from making filled and coated paper products. The present invention uses a composition comprising kaolin having a shape factor of at least about 70:1, such as at least about 80:1 or at least about 100:1.

RELATED APPLICATION

This is a divisional of application Ser. No. 10/233,599, filed Sep. 4,2002, now abandoned, which claims the benefit of priority of U.S.Provisional Application No. 60/318,207, filed Sep. 7, 2001, both ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a hydrous kaolin, which providesimproved optical properties when used as a filler or coating. Also, thepresent invention relates to a paper coating and a coated paper productthat exhibits improved properties, such as improved brightness. Further,the present invention relates to methods for preparing hydrous kaolinproducts.

BACKGROUND OF THE INVENTION

Particulate kaolin products find a variety of uses, including aspigments, fillers, and extenders for use in paint, plastics, polymers,papermaking and paper coating. Kaolin clay, also referred to as ChinaClay, or hydrous kaolin, is comprised predominantly of the mineralkaolinite, a hydrous aluminum silicate, together with small amounts of avariety of impurities.

Particulate kaolins generally exist in three forms: hydrous kaolin,calcined kaolin and chemically aggregated kaolin. Hydrous kaolin isprimarily the mineral kaolinite, which has been mined and beneficiatedfrom natural sources. Calcined kaolins are obtained by processinghydrous kaolin at high temperatures, e.g., greater than 500° C.Chemically aggregated kaolins are particle aggregates having amicrostructure resembling that of calcined kaolins produced by treatinghydrous kaolin with chemicals. Calcined and chemically aggregatedkaolins can show benefits in certain application compositions whencompared with hydrous kaolins. However, the benefits associated withcalcined and chemically aggregated kaolins are not withoutdisadvantages. The manufacturing costs of calcined and chemicallyaggregated kaolins are significantly higher than those of hydrouskaolins. The calcined and chemically aggregated kaolins also have theeffect of improving certain paper properties while adversely effectingother properties, e.g., strength.

Kaolin has been used as an extender or pigment in paints, plastics andpaper coating compositions. Kaolin pigments confer desirable physicaland optical properties to such compositions. As flattening (or matting)agents, they help smooth the surfaces of the substrates to which theyare applied. As opacifiers, they impart brightness, whiteness, gloss andother desirable optical properties. As extenders, they allow partialreplacement of titanium dioxide and other more expensive pigments withminimal loss of whiteness or brightness.

Paper coatings are applied to sheet materials for a number of purposesincluding, but not limited to, increasing the gloss, smoothness, opacityand/or brightness of the material. Coatings may also be applied to hidesurface irregularities or in other ways improve the surface for theacceptance of print. Paper coatings are generally prepared by forming afluid aqueous suspension of pigment material together with a hydrophilicadhesive and other optional ingredients.

Coatings have been conventionally applied by means of a coating machineincluding a short dwell time coating head, which is a device in which acaptive pond of coating composition under a slightly elevated pressureis held in contact with a moving paper web for a time sufficient to coatthe paper before excess coating composition is removed by means of atrailing blade.

Additional advantages of the invention will be set forth in part in thedescription which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention will be realized and attained by meansof the elements and combinations particularly pointed out in theappended claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed.

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments of theinvention and together with the description, serve to explain theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating a process for refining a coarsefraction kaolin according to Example 1;

FIG. 2 is a flowchart illustrating a process for refining a coarsefraction kaolin according to Example 2;

FIG. 3 is a flowchart illustrating a process for refining a coarsefraction kaolin according to Example 3;

FIG. 4 is a flowchart illustrating the process for making a barriercoating pigment from a coarse fraction kaolin;

FIG. 5 is a graph illustrating brightness (y-axis) as a function of theTiO₂ content (x-axis) for two paper coatings according to the presentinvention;

FIG. 6 is a graph illustrating sheen (y-axis) versus opacity (x-axis) ofvarious ground U.S. clays;

FIG. 7 illustrates a plot of the d50 (y-axis) versus shape factor(x-axis) for Invention Kaolin 3 and Invention Kaolin 4 compared to twoprior art kaolins;

FIG. 8 is a flowchart illustrating a process for refining a coarsefraction kaolin according to Example 10.

SUMMARY OF THE INVENTION

Generally, kaolins used in coatings and fillers are selected to have anarrow particle size distribution and high crystallinity which have beenbelieved to provide the favored set of physical and optical properties,for example, maximum light scatter. Generally, coarse kaolin clays havebeen recognized as exhibiting poor light scatter. The present inventiondiffers from the prior art in providing a coating or filling compositionincluding a kaolin that may be rather coarse, but which nonethelessexhibits high light scatter. As a filler in, for example, unbleachedkraft paper, the kaolins according to the present invention showed alight scatter similar to calcined clays without the detrimental effectson strength associated with calcined clays. The compositions accordingto the present invention further improve over prior compositions bymaking it possible to maintain sheet brightness while further reducingthe amount of expensive TiO₂ present in the paper coating composition.

There is disclosed a paper coating composition comprising at least onekaolin having a shape factor of at least about 70:1, a TiO₂, a binder,and optionally a dispersant, and wherein the paper coating compositionhas a ratio of the kaolin to TiO₂ of less than about 85:15 parts byweight.

There is further disclosed a method of making a coated paper comprisingcoating a fibrous substrate with a paper coating composition comprisinga binder and at least one kaolin having a shape factor of at least about70:1.

There is still further disclosed a method of making a filled papercomprising adding to said paper kaolin having a shape factor of at leastabout 70:1

There is also disclosed a coated paper comprising a fibrous substrateand a paper coating composition comprising kaolin having a shape factorof at least about 70:1.

Finally, there is disclosed a method of improving barrier properties inpaper comprising coating a fibrous substrate with a coating compositioncomprising kaolin having a shape factor of at least about 70:1.

One aspect of the present invention provides a composition comprisingkaolin having a shape factor of at least about 70:1. In otherembodiments, the kaolin can have a shape factor of at least about 80:1,at least about 90:1, or at least about 100:1.

Another aspect of the present invention provides a coating composition,comprising kaolin having a shape factor of at least about 70:1, abinder; and optionally a dispersant.

Another aspect of the present invention provides a filler comprisingkaolin having a shape factor of at least about 70:1.

Another aspect of the present invention provides a method of making acoated paper comprising coating a fibrous substrate with a paper coatingcomposition comprising a carrier and kaolin having a shape factor of atleast about 70:1.

Another aspect of the present invention provides a method of making afilled paper comprising providing a fibrous substrate, and adding to thesubstrate kaolin having a shape factor of at least about 70:1.

Another aspect of the present invention provides a coated papercomprising a fibrous substrate and a coating on the substrate comprisingkaolin having a shape factor of at least about 70:1.

Another aspect of the present invention provides a method of improvingbarrier properties in paper comprising coating a fibrous substrate witha coating composition comprising kaolin having a shape factor of atleast about 70:1.

Another aspect of the present invention provides a method of refiningkaolin, comprising providing a kaolin slurry, attrition grinding thekaolin slurry, removing the coarse particle size fraction from theslurry by centrifugation, and attrition grinding the coarse-particlesize fraction to provide a kaolin product having a shape factor of atleast about 70:1.

Another aspect of the present invention provides an extender for paintcomprising kaolin, prepared by a method comprising providing a kaolinslurry comprising whole crude kaolin or a blend of whole crude andcoarse-particle size fractions from a centrifuge, attrition grinding thekaolin slurry, removing the coarse particle size fraction from theslurry by centrifugation, and attrition grinding the coarse-particlesize fraction to provide a kaolin product having a shape factor of atleast about 70:1.

Another aspect of the present invention provides a slurry comprisingkaolin having a shape factor of greater than 70:1 and a stabilizingagent.

DETAILED DESCRIPTION

One aspect of the present invention provides a composition that is usedin the production of coated or filled substrates. Another aspect of thepresent invention provides the coated or filled products made accordingto the present invention. The compositions and products according to thepresent invention utilize kaolin having a high shape factor to prepare acoated or filled paper exhibiting improved brightness. “Shape factor” asused herein is a measure of an average value (on a weight average basis)of the ratio of mean particle diameter to particle thickness for apopulation of particles of varying size and shape as measured using theelectrical conductivity method and apparatus described in U.S. Pat. No.5,128,606, which is incorporated herein by reference in its entirety. Inthe measurement method described in U.S. Pat. No. 5,128,606, theelectrical conductivity of a fully dispersed aqueous suspension of theparticles under test is caused to flow through an elongate tube.Measurements of the electrical conductivity are taken between (a) a pairof electrodes separated from one another along the longitudinal axis ofthe tube, and (b) a pair of electrodes separated from one another acrossthe transverse width of the tube, and using the difference between thetwo conductivity measurements the shape factor of the particulatematerial under test is determined. “Hyperplaty” refers to hydrous kaolinclays with shape factors of greater than about 40:1. In one embodimentaccording to the present invention, the kaolin clays have shape factorsof greater than about 70:1, for example greater than about 80:1, orgreater than about 90:1. According to another aspect of the invention,the shape factor is greater than about 100:1, for example 110:1 andabove. In another embodiment, the kaolin clay has a shape factor of atand above about 120:1 and at and above about 140:1. The clays accordingto these embodiments of the invention have been found to showsurprisingly high light scatter in paper coatings and fillings.

The high shape factor may be achieved by grinding mined kaolinitic claysuntil the desired shape factor is achieved. Any art recognized grindingmethod can be used with the present invention including but not limitedto, for example, wet grinding using sand or ceramic media. According toone embodiment of the present invention, the kaolin may be prepared bylight comminution, e.g., grinding or milling, of a coarse kaolin to givesuitable delamination thereof. The comminution may be carried out by useof beads or granules of a ceramic or plastic, e.g., nylon, grinding ormilling aid. Appropriate grinding energies will be readily apparent andeasily calculated by the skilled artisan. Significant grinding energiesmay be necessary to attain desirable high shape factors, however kaolincrude selected for its natural platyness will grind to high shapefactors in an energy range typically used to manufacture standarddelaminated kaolin pigments that have lesser shape factors.

Crude kaolin or a high shape factor product obtained from grinding ormilling may be refined to remove impurities and improve physicalproperties using well known procedures generally referred to asbeneficiation processes. The kaolin may be treated by a known particlesize classification procedure, screening and/or centrifuging, to obtainparticles having a desired particle size distribution and d₅₀ value (asdiscussed below). According to one embodiment according to the presentinvention, mined clays are suitably first degritted before they aresubjected to grinding to achieve the desired shape factor.

“Mean particle diameter” is defined as the diameter of a circle that hasthe same area as the largest face of the particle. The mean particlesize, d₅₀ value, and other particle size properties referred to in thepresent application are measured in a well known manner by sedimentationof the particulate material in a fully dispersed condition in an aqueousmedium using a SEDIGRAPH 5100 machine as supplied by MicromeriticsCorporation. Such a machine provides measurements and a plot of thecumulative percentage by weight of particles having a size, referred toin the art as the “equivalent spherical diameter” (esd), less than thegiven esd values. The mean particle size d₅₀ is the value determined inthis way of the particle esd at which there are 50% by weight of theparticles, which have an esd less than that d₅₀ value.

The value of d₅₀ for the particulate kaolin according to the presentinvention may be, for example, in the range of about 0.2 μm to about 10μm.

Previously, it was believed that coarse kaolin particles yielded pigmentproducts having poor light scatter. The present invention's use ofhyperplaty clay allows significantly coarser products to be used withcomparable or better light scatter. According to one embodiment of thepresent invention, the particle size distribution of the kaolin may besuch that about 20% or more of the particles have an esd of 2 microns orless. A suitable product according to the invention may have a particlesize distribution where about 50% to about 85% of the particles have anesd of 2 microns or less.

The steepness of the particle size distribution (psd) of the particulatekaolin according to the present invention, often referred to asnarrowness of the psd, refers to the slope of the psd curve. Thus, insome cases the psd of the kaolin according to the present invention maybe steep and in other cases it may be broad. The steepness or broadnessof the psd is one indicator of anticipated product performance and canbe selected by the skilled artisan based upon desired end properties.Steepness, as used in herein, is measured as 100 times the ratio of d₃₀to d₇₀, where d₃₀ is the value of the particle esd less than which thereare 30% of the particles and d₇₀ is the value of the particle esd lessthan which there are 70% of the particles as obtained from the psdmeasure above.

Another aspect of the present invention provides a kaolin product havinga shape factor value of at least about 70 formed by attrition grinding acoarse particle size kaolin fraction. Kaolin consists predominantly ofkaolinite crystals, which are shaped as thin hexagonal plates or inbooklets of platelets called “stacks.” Kaolinite stacks may be subjectedto a grinding action to easily separate or delaminate the stacks orbooks comprised of more than one platelet into smaller books orindividual platelets. The act of delamination parts or cleaves naturalkaolinite crystals along the (001) crystallographic plane that isperpendicular to its “c-axis.” Many standard kaolin products are made byattrition grinding a dispersed slurry of blunged-degritted-brightnessbeneficiated blend of whole crude and coarse-particle size fraction froma centrifuge. These standard delaminated products, however, may not showsufficient delamination, as they can contain booklets that compriseseveral to scores of kaolinite platelets.

Accordingly, this aspect of the present invention provides a method forobtaining a kaolin having a shape factor of about 70:1 or higher. Themethod comprises: (a) providing a kaolin slurry, where in oneembodiment, the slurry comprises a whole crude kaolin or a blend ofwhole crude and coarse particle kaolin from a centrifuge; (b) attritiongrinding the kaolin slurry; (c) removing a coarse particle size fractionfrom the slurry by centrifugation; and (d) attrition grinding thecoarse-particle size fraction. Optionally, a centrifugation step may beinserted prior to (b) to remove ultrafine particles in order to enhancethe efficiency of attrition grinding.

In one embodiment, the kaolin product is produced by attrition grindinga crude clay slurry. The crude can comprise, in one embodiment, either awhole crude or a blend of whole crude and coarse-particle size fractionsfrom a centrifuge. The clay slurry may be adispersed-degritted-brightness beneficiated clay slurry.

In one embodiment, the kaolin slurry can be obtained from crude kaolinore, which is generally a viscous, sedimentary kaolin containing micas,mixed-layered clay minerals, smectites, and vermiculites or hydrousmicas. An exemplary kaolin clay crude further comprises potassium oxidein an amount ranging from 0.10% to 3.0% by weight, and magnesium oxidein an amount ranging from 0.03% to 0.5% by weight. Such crude can have ashape factor equal to or greater than 15, or equal to or greater than25. The crude kaolin can have a particle size distribution of 75% byweight less than 2 μm.

To form the kaolin slurry, water can be added to a dry kaolin crude toform an aqueous suspension, such as a suspension containing from 30% to70% kaolin on a dry weight basis. Chemicals can be added to disperse theclay particles. The kaolin slurry can then be degritted to removesand-sized particles prior to further processing or transport bypipeline to a plant.

Where the crude kaolin slurry comprises a blend of whole crude andcoarse-particle size fractions from a centrifuge, the coarse particlesize fraction may have a shape factor of greater than 20, such as ashape factor greater than 25. In another embodiment, the coarse particlesize fraction may have a shape factor of greater than 35.

The attrition grinding steps can be performed with a particulategrinding medium for a time sufficient to dissipate in the slurry orsuspension enough energy to impart the kaolin particles with a shapefactor value ranging from about 35 to about 60 or greater. In anotherembodiment, the attrition grinding the crude kaolin slurry results inkaolin having a shape factor value of at least about 60.

The amount of energy dissipated in the suspension of kaolin clay canrange from about 20 kWh to about 100 kWh, such as from about 25 to about75 kWh of energy per ton of kaolin present on a dry weight basis. Theattrition grinding mill is equipped with a stirrer, which is capable ofbeing rotated at a speed such that a vortex is formed in the suspensionin the mill during grinding.

The particulate grinding medium can have a specific gravity of 2 ormore. In one embodiment, the grinding medium comprises grains of quartzsand, or similar media. The grains can have a diameter of less than 2mm. In another embodiment, the grains have a diameter of greater than0.25 mm and less than 2 mm. In yet another embodiment, the grindingmedium comprises fine (20-40#) Carbolite media.

Prior to attrition grinding the crude kaolin slurry, the slurry can besubjected to beneficiation and/or degritting processes. In oneembodiment, the crude kaolin slurry comprises adispersed-degritted-brightness beneficiated clay slurry. Degritting isthe process of passing a kaolin clay slurry through drag boxes and ascreen to remove coarse (grit) particles of a given size. For example,the screen can be a +325 mesh (U.S. Standard) screen suitable forremoving particles larger than 45 μm.

After attrition grinding the crude slurry, the slurry can be passedthrough a centrifuge, such as a solid-bowl decanter centrifuge, for aclassification step, where the clay is classified to a particle sizedistribution such that 80% to 95% by weight of the particles have an esdless than 2 μm. In one embodiment, 85% to 92% by weight of the particleshave an esd less than 2 μm. In yet another embodiment, 20% to 40% of theparticles have an esd less than 0.25 μm. In still another embodiment,25% to 35% of the particles have an esd less than 0.25 μm. In stillanother embodiment, the weight of the particles in the finer fractionranges from 5% to 30% of the feed to centrifuge.

After removing the coarse-particle size fraction of the attrition-groundproduct, i.e. the “b-fraction” via a centrifuge, the coarse-fraction canbe diluted with water to form a slurry that may be subjected to anadditional attrition grinding step to produce the final kaolin producthaving a shape factor value of at least 70. This product mayalternatively subject to further beneficiation to obtain desiredbrightness or rheology through conventional processing techniques suchas flotation, selective flocculation, and bleaching. Previously, in someinstances, course fractions were discarded, as this kaolin fraction wassometimes regarded as being too coarse for coating applications. Thus,one advantage of this embodiment arises from the use of coarse fraction,which was in some cases a previously undesired manufacturing by-product,to produce a useful, kaolin product.

In one embodiment, the coarse particle size fraction comprises kaolinhaving a shape factor of at least about 20, such as at least about 25 orat least about 30, and in some cases as high as about 50 to about 60.

The final kaolin particles can have a mean particle size (d₅₀ bySedigraph) ranging from about 0.1 μm to about 2.0 μm, such as a d₅₀ranging from about 0.25 μm to about 1 μm.

In another embodiment the coarse particle fraction kaolin slurry issubjected to a beneficiation step and/or a degritting step, prior to theattrition grinding process. In another embodiment, the coarse particlefraction kaolin slurry is subjected to both beneficiation and degrittingprocesses.

In one embodiment, the invention can advantageously reduce thepopulation of kaolin crystals that exist as stacks. The kaolin productof the invention can have superior coverage relative to standard kaolinin lightweight and ultra lightweight coated paper applications, as thehigher population of individual plate crystals present in hyperplatykaolin provides a relatively thin and uniform plate thickness. In oneembodiment, the invention provides a paper coated with hyperplatykaolin, as described herein.

Another aspect of the present invention provides an extender for paints,the extender comprising a coarse particle size kaolin having a d₅₀ranging from about 0.25 μm to about 2.0 μm. In one embodiment, thekaolin extender has a shape factor of greater than about 90. Theextender can be prepared by attrition grinding a coarse particle sizekaolin fraction, as described above.

In certain applications, it may be desired to use extenders with abalance of low sheen and high opacity. Accordingly, in one embodiment,the extender is coarse, with a d₅₀ ranging from 0.25 μm to 2.0 μm, suchas a d₅₀ ranging from 0.5 μm to 1.5 μm. For example, the d₅₀ can beapproximately 1.3 μm. In other applications, control of sheen may not berequired, and therefore the kaolin can comprise finer particles with ad₅₀ ranging from 0.25 μm to 1 μm. For example, the d₅₀ can beapproximately 0.6 μm.

Another embodiment provides a paint comprising an extender comprising acoarse particle size kaolin having a d₅₀ ranging from 0.25 μm to 2.0 μm.In one embodiment, the kaolin extender has a shape factor of at least70:1, or any other value described herein. In another embodiment, thekaolin extender has a shape factor of at least 100:1.

Another aspect of the present invention provides an extender for paintcomprising a kaolin, prepared by a method comprising providing a kaolinslurry comprising whole crude kaolin or a blend of whole crude andcoarse-particle size fractions from a centrifuge, attrition grinding thekaolin slurry, removing the coarse particle size fraction from theslurry by centrifugation, and attrition grinding the coarse-particlesize fraction to provide a kaolin product having a shape factor of atleast about 70:1, or any of the shape factors described herein.

Another aspect of the present invention relates to the use of thehyperplaty clay for filling, for example, paper or paperboard product,such as a supercalendered magazine paper filler product. In oneembodiment, the paper or paperboard product is filled with a kaolinhaving a shape factor value of at least about 70.

Another aspect of the invention provides a method of making a mattepaper coating, comprising: (a) attrition grinding a crude kaolin slurrycomprising a blend of coarse-fractions from a centrifuge that have beendegritted to 80% finer than 5 μm; (b) removing the coarse particle sizefraction from the slurry by centrifugation; and (c) attrition grindingthe coarse particle size fraction to provide kaolin having a shapefactor of at least 70:1.

Another aspect of the invention provides a method for producing apigment product comprising:

(a) mixing a raw or partially processed kaolin clay with water to forman aqueous suspension;

(b) attrition grinding the suspension produced by step (a) by using aparticulate grinding medium by a process in which the average shapefactor of the kaolin clay is increased by at least 10, preferably atleast 20;

(c) separating the suspension of ground kaolin clay from the particulategrinding medium; and

(d) dewatering the suspension of ground kaolin clay separated in step(c) to recover a kaolin pigment therefrom.

In step (a) of the method according to the second aspect of the presentinvention, the kaolin clay may form from 20% to 70%, usually from 20% to45% of the treated suspension. The kaolin clay can comprise asedimentary kaolin clay, such as a sedimentary kaolin clay from Georgia,USA. The raw kaolin clay may have a psd such that not more than about40% by weight comprises particles having an esd larger than 10 μm andnot more than 50% by weight, e.g. from about 20% to about 40% by weight,comprising particles having an esd smaller than 2 μm. The shape factorof the kaolin clay treated in step (a) may be less than 15, such as inthe range of from about 5 to about 10. Thus, the shape factor may beincreased by a differential of at least 30, in some cases at least 40,e.g. from a shape factor value of less than 15 to a shape factor valuegreater than 55.

When preparing an aqueous suspension of the kaolin clay to be treated instep (a) a dispersing agent for the kaolin clay may or may not be addedto the kaolin clay.

The kaolin clay employed in step (a) may be a coarse component obtainedfrom classifying, e.g. using a centrifuge, a standard blocky sedimentarykaolin clay, such as a kaolin clay having a shape factor of from 5 to10. The coarse component may have not more than 50% by weight ofparticles having an esd less than 2 μm and not more than 10% by weighthaving an esd less than 0.25 μm.

The psd of the kaolin clay may be adjusted by blending from 99 to 50parts by weight of the kaolin clay with from 1 to 50 parts by weight,such as from 10 to 30 parts by weight, of a fine platy kaolin component,e.g. having a shape factor of at least 15, such as from 15 to 40 andwhose percentages by weight of particles smaller than 2 μm and 0.25 μmare respectively at least 85% by weight and at least 20% by weight. Thefine platy kaolin component may be a kaolin derived from either aprimary or a sedimentary deposit. The fine platy kaolin component may beadded to the kaolin or obtained from the coarse component prior to orafter the grinding step (b). The addition may be carried out with thekaolins to be blended in either powdered, dry form or in the form of anaqueous suspension.

A resulting kaolin product prepared by blending in the manner describedcan improve characteristics of the kaolin, such as the rheology anddewatering characteristics of a resulting aqueous suspension, and canprovide better runnability and particle alignment when the kaolin isused in a coating composition.

The kaolin clay may be subjected to one or more well known purificationsteps to remove undesirable impurities, e.g. between steps (a) and (b)or between steps (c) and (d). For example, the aqueous suspension ofkaolin clay may be subjected to a froth flotation treatment operation toremove titanium containing impurities in the froth. Alternatively, or inaddition, the suspension may be passed through a high intensity magneticseparator to remove iron containing impurities.

Step (b) may comprise a process wherein the suspension of kaolin clay istreated by medium attrition grinding wherein an energy of from about 40kWh to about 250 kWh per tonne of clay (on a dry weight basis) isdissipated in the suspension.

The process of step (b) may comprise a process comprising at least twostages, namely a first stage (b1) wherein delamination of the kaolinclay occurs and a second stage (b2) wherein comminution of the plateletsof the kaolin clay occurs.

A gentle comminution step (b1) can be performed, such as grinding via aparticulate grinding medium in order to break down composite particles,which are present in the raw kaolin clay. Such composite particlesgenerally comprise coherent stacks or blocks of individual hexagonalplate-like particles, especially where the kaolin clay is from asedimentary deposit. When the kaolin clay is subjected to relativelygentle comminution, e.g. by grinding in step (b1), many of the compositeparticles can be broken down to give the individual thin, substantiallyhexagonal plates, i.e., delamination, the result being an increase ofthe average shape factor of the kaolin clay. For example, this processmay increase the shape factor of the kaolin clay from a starting shapefactor of about 5 to 10 to an increased shape factor of at least about50 to 55. By “relatively gentle grinding” we mean grinding in anattrition grinding mill with a particulate grinding medium, the contentsof the attrition grinding mill being agitated by means of an impellerwhich rotates at a speed which is insufficient to set up a vortex in thesuspension, such as at a peripheral speed below about 10 m.s⁻¹ and inwhich the amount of energy dissipated in the suspension during grindingis less than about 75 kWh, such as less than about 55 kWh per ton ofkaolin clay on a dry weight basis. The particulate grinding medium canbe of relatively high specific gravity, for example 2 or more, and mayfor example may comprise grains of silica sand, the grains generallyhaving diameters not larger than about 2 mm and not smaller than about0.25 mm.

The second stage (b2) of the two stage form of step (b) can comprisegrinding in an attrition grinding mill which is equipped with a stirrercapable of being rotated at a speed such that a vortex is formed in thesuspension in the mill during grinding. The particulate grinding mediumcan have a specific gravity of 2 or more, and can comprise, for example,grains of silica sand where the grains can have diameters not largerthan about 2 mm and not smaller than about 0.25 mm. If stage (b2) ispreceded by a relatively gentle comminution in stage (b1), the amount ofenergy dissipated in the suspension of kaolin clay in stage (b2) can bein the range of from about 40 kWh to about 120 kWh per dry ton of kaolinclay. If the relatively gentle comminution step (b1) is omitted, theamount of energy dissipated in the suspension of kaolin clay in step (b)can be in the range of from about 100 kWh to about 250 kWh per dry tonof kaolin clay.

In step (c), the suspension of ground kaolin clay can be separated fromthe particulate grinding medium in any manner known in the art, such bypassing the suspension through a sieve of appropriate aperture size, forexample a sieve having nominal aperture sizes in the range of from about0.1 mm to about 0.25 mm.

Following step (c) or step (d) the kaolin clay may be further treated toimprove one or more of its properties. For example high energy liquidworking, such as by using a high speed mixer, may be applied to theproduct in slurry form, e.g. before step (d) or after step (d) andsubsequent re-dispersion in an aqueous medium, e.g. during makedown of acoating composition.

In step (d), the suspension of ground kaolin may be dewatered in one ofthe ways well known in the art, e.g. filtration, centrifugation,evaporation and the like. In one embodiment, dewatering can be performedvia a filter press. For example, use of a filter press may be made toform a cake having a water content in the range of from about 15% toabout 35% by weight. This cake may be mixed with a dispersing agent forthe kaolin clay and can be converted into a fluid slurry, which may betransported and sold in this form. Alternatively, the kaolin clay may bethermally dried, for example by introducing the fluid slurry of thekaolin clay into a spray drier and transported in a substantially dryform.

A fine platy kaolin component may be blended with the product of step(d) in the manner described earlier if such a component has not alreadybeen added prior to step (d).

In one embodiment, the pigment product may have a specific surface areaas measured by the BET, N₂ method of at least 12 m².g⁻¹, preferably from15 m².g⁻¹ to 20 m².g⁻¹.

In one embodiment, pigment product produced in this aspect of theinvention may be used in paper coating. The coating composition can beused for producing gloss coatings on paper and other substrates whichcomposition comprises an aqueous suspension of a particulate pigmenttogether with a hydrophilic adhesive or binder.

The solids content of the paper coating composition may be greater than60% by weight, such as a solids content of at least 70%, or even as highas possible but still giving a suitably fluid composition which may beused in coating. The composition may include a dispersing agent, e.g. upto 2% by weight of a polyelectrolyte based on the dry weight of pigmentpresent, or any other dispersing agent known in the art, such as thosedescribed herein. The pigment product according to the first aspect ofthe invention may be used as the sole pigment in a paper coatingcomposition, or it may be used in conjunction with one or more otherknown pigments, such as (commercially available) kaolin, calcinedkaolin, natural or precipitated calcium carbonate, titanium dioxide,calcium sulphate, satin white, talc and so called ‘plastic pigment’. Inone embodiment, when a mixture of pigments is used, the pigment productaccording to this aspect of the invention, is present in the mixture inan amount of at least 80% of the total dry weight of the mixed pigments.

The binder of the composition may comprise an adhesive derived fromnatural starch obtained from a known plant source as described herein,although it is not essential to use starch as a binder ingredient. Otherbinders, which may be used with or without starch are also describedherein.

Where starch is employed as a binder ingredient, the starch may beunmodified or raw starch, or it may be modified by one or more chemicaltreatments known in the art. The starch may, for example, be oxidized toconvert some of its —CH₂OH groups to —COOH groups. In some cases thestarch may have a small proportion of acetyl, —COCH₃, groups.Alternatively, the starch may be chemically treated to render itcationic or amphoteric, i.e., with both cationic and anionic charges.The starch may also be converted to a starch ether, or hydroxyalkylatedstarch by replacing some —OH groups with, for example, —OCH₂CH₂OHgroups, —OCH₂CH₃ groups or —OCH₂CH₂CH₂OH groups. A further class ofchemically treated starches that may be used is that known as the starchphosphates. Alternatively, the raw starch may be hydrolyzed by means ofa dilute acid or an enzyme to produce a gum of the dextrin type.

The starch binder used in the composition according to this aspect ofthe invention can be present in an amount ranging from 4% to 25% byweight, based on the dry weight of pigment. The starch binder may beused in conjunction with one or more other binders, for examplesynthetic binders of the latex or polyvinyl acetate or polyvinyl alcoholtype. When the starch binder is used in conjunction with another binder,e.g. a synthetic binder, the amount of the starch binder can be presentin an amount ranging from 2% to 20% by weight, and the amount of thesynthetic binder can be present in an amount ranging from 2% to 12% byweight, both based on the weight of dry pigment. In one embodiment, atleast 50% by weight of the binder mixture comprises modified orunmodified starch.

Another aspect of the present invention provides a method of forming acoated paper, comprising applying the kaolin composition to coat a sheetof paper and calendering the paper to form a gloss coating thereon. Inone embodiment, the gloss coating is formed on both sides of the paper.

Another aspect of the present invention provides a method of making acoated paper comprising, coating a fibrous substrate with a papercoating composition comprising a filler comprising kaolin having a shapefactor of at least about 70:1, or any of the shape factors describedherein.

Another aspect of the present invention provides a coated papercomprising a fibrous substrate and a coating on the substrate comprisingkaolin having a shape factor of at least about 70:1, or any of the shapefactors described herein.

Another aspect of the present invention provides a method of making afilled paper comprising providing a fibrous substrate and adding to thesubstrate kaolin having a shape factor of at least about 70:1, or any ofthe shape factors described herein.

Calendering is a well known process in which paper smoothness and glossis improved and bulk is reduced by passing a coated paper sheet betweencalender nips or rollers one or more times. Usually, elastomer coatedrolls are employed to give pressing of high solids compositions. Anelevated temperature may be applied. Five or more passes through thenips may be applied.

The paper after coating and calendering in the method according to thisaspect may have a total weight per unit area ranging from 30 g.m⁻² to 70g.m⁻², such as from 49 g.m⁻² to 65 g.m⁻², or further from 35 g.m⁻² to 48g.m⁻². The final coating can have a weight per unit area ranging from 3g.m⁻² to 20 g.m⁻², such as from 5 g.m⁻² to 13 g.m⁻². Such a coating maybe applied to both sides of the paper. The coated paper may be LWC orULWC paper. The paper gloss may be greater than 45 TAPPI units and theParker Print Surf value at a pressure of 1 MPa of each paper coating maybe less than 1 μm.

The gloss of a coated paper surface may be measured by means of a testlaid down in TAPPI Standard No 480 ts-65. The intensity of lightreflected at an angle from the surface of the paper is measured andcompared with a standard of known gloss value. The beams of incident andreflected light are both at an angle of 75° to the normal to the papersurface. The results are expressed in TAPPI gloss units. In oneembodiment, the gloss of the pigment product may be greater than 50, insome cases greater than 55, TAPPI units.

The Parker Print Surf test provides a measure of the smoothness of apaper surface, and comprises measuring the rate at which air underpressure leaks from a sample of the coated paper which is clamped, undera known standard force, between an upper plate which incorporates anoutlet for the compressed air and a lower plate, the upper surface ofwhich is covered with a sheet of either a soft or a hard referencesupporting material according to the nature of the paper under test.From the rate of escape of the air, a root mean cube gap in μm betweenthe paper surface and the reference material is calculated. A smallervalue of this gap represents a higher degree of smoothness of thesurface of the paper under test.

An improvement can be provided where the binder present in the coatingcomposition comprises starch. However, an improvement is also obtainedwhere other known starch-free binders are employed (with or withoutstarch present). In each case the adhesive or binder may form from 4% to30%, e.g., 8% to 20%, for example from 8% to 15% by weight of the solidscontent of the composition. The amount employed will depend upon thecomposition and the type of adhesive, which may itself incorporate oneor more ingredients

The particulate kaolin according to the present invention may be mixedwith other pigments, fillers and extenders to obtain a blend ofproperties provided by the constituents of the mixture. The addedpigment, filler or extender material may comprise one or more of hydrouskaolin, calcined kaolin, aggregated kaolin, calcium carbonate (ground orprecipitated) talc, gypsum or other known white particulate mineral orpigment material.

Paper coatings and paper fillers according to the present inventioninclude, in addition to the kaolin as described above, materialsgenerally used in the production of paper coatings and paper fillers.Specifically, the compositions will include a binder and a pigment,typically TiO₂. The fillers and coatings according to the presentinvention may optionally include other additives, including, but notlimited to, dispersants, cross linkers, water retention aids, viscositymodifiers or thickeners, lubricity or calendering aids,antifoamers/defoamers, gloss-ink hold-out additives, dry or wet rubimprovement or abrasion resistance additives, dry or wet pickimprovement additives, optical brightening agents or fluorescentwhitening agents, dyes, biocides, leveling or evening aids, grease oroil resistance additives, water resistance additives and/orinsolubilisers.

Any art recognized binder may be used in the present invention.Exemplary binders include, but are not limited to, adhesives derivedfrom natural starch obtained from a known plant source, for example,wheat, maize, potato or tapioca; synthetic binders, including styrenebutadiene, acrylic latex, vinyl acetate latex, or styrene acrylic;casein; polyvinyl alcohol; polyvinyl acetate; or mixtures thereof.

Paper coatings have very different binder levels depending upon the typeof printing to be used with the coated paper product. Appropriate binderlevels based upon the desired end product would be readily apparent tothe skilled artisan. Binder levels are controlled to allow the surfacesto receive ink without disruption. The latex binder levels for papercoatings generally range from about 3% to about 30%. In one embodimentaccording to the present invention, the binder is present in the papercoating in an amount of from about 3% to about 10%. In anotherembodiment according to the present invention, the binder is present inthe coating in an amount ranging from about 10% to about 30% by weight.

Another aspect of the present invention provides a method of making abarrier coating from a coarse fraction kaolin having the propertiesdescribed herein. Barrier coatings are useful to impart to paperresistance to moisture, moisture vapor, grease, oil, air, etc.

When making barrier coatings, the amount of binder in the formulationmay be very high on the order of 40% to 50%. The binder combined withthe flattest possible plates result in a coating, which exhibitsexcellent barrier properties.

Another aspect of the present invention provides a barrier coatingcomposition, comprising a slurry comprising kaolin having a shape factorof at least about 70:1, or any of the shape factors described herein.The solids content of the slurry can range from about 45% to about 70%.

Another aspect of the present invention provides a method of improvingbarrier properties in a paper comprising coating a fibrous substratewith a paper coating composition comprising a kaolin having a shapefactor of at least about 70:1, or any of the shape factors describedherein.

Another aspect of the present invention provides a filler comprising akaolin having a shape factor of at least about 70:1, or any of the shapefactors described herein.

Another aspect of the present invention provides a slurry comprising akaolin having a shape factor of greater than 70:1 and a stabilizingagent. In one embodiment, the kaolin can have a shape factor of at leastabout 80:1, or any of the shape factors described herein.

Many stabilizing agents are known in the art for such slurries. In oneembodiment, the stabilizing agent comprises carboxymethylcellulose. Inanother embodiment, the stabilizing agent can comprise a smectite clay.In yet another embodiment, the stabilizing agent can comprise bentonite.In still another embodiment, the stabilizing agent can comprisehectorite.

In one embodiment, the stabilizing agent is present in the slurry in anamount ranging from about 4 pounds per dry tonne kaolin to about 10 lbsper dry tonne kaolin.

In one embodiment, the slurry has a solids content of greater than about45%, such as a solids content ranging from about 50% to about 60%.

Dispersants may be chosen from any art recognized dispersants for use inpaper coating or paper filling compositions. Appropriate dispersantswill be readily apparent to the skilled artisan. Dispersant may bechosen from polyelectrolytes such as polyacrylates and copolymerscontaining polyacrylate species, especially polyacrylate salts (such assodium and aluminum optionally with a group II metal salt), sodiumhexametaphosphates, non-ionic polyol, polyphosphoric acid, condensedsodium phosphate, non-ionic surfactants, alkanolamine and other reagentscommonly used for this function.

The kaolin according to the present invention may be used as the solepigment in the paper coatings and paper fillers of the presentinvention. Alternatively, the hyperplaty kaolin described above may becombined with another pigment. Additional pigments for use in thecomposition according to the present invention can be selected from anyart recognized pigment. Pigments may be selected from other kaolin,calcined kaolin, natural or precipitated calcium carbonate, titaniumdioxide, calcium sulphate, satin white, talc or other plastic pigments.According to the present invention, a pigment can be TiO₂. In exemplarycompositions according to the present invention, TiO₂ is present in anamount ranging from 0 to 30 parts, such as from 5–20 parts or from 10–20parts.

If cross linkers are used in the coating or filling composition for usewith the present invention, crosslinkers are generally present in levelsof up to 5% by weight. Any art recognized crosslinker may be used.Appropriate crosslinkers include, but are not limited to, glyoxals,melamine formaldehyde resins, ammonium zirconium carbonates and mixturesthereof.

If a water retention aid is used in the coating or filling compositionfor use with the present invention, water retention aids are generallypresent up to 2% by weight. Any art recognized water retention aid maybe used. Appropriate water retention aids include, but are not limitedto, sodium carboxymethyl cellulose, hydroxyethyl cellulose, PVA(polyvinyl acetate), starches, proteins, polyacrylates, gums, alginates,polyacrylamide bentonite and other commercially available products soldfor such applications.

If a viscosity modifier or thickener is used in the coating or fillingcomposition of the present invention, it is generally present in levelsup to 2% by weight. Any art recognized thickener or viscosity modifiermay be used. Appropriate viscosity modifiers or thickeners include, butare not limited to, polyacrylates, emulsion copolymers, dicyanamide,triols, polyoxyethylene ether, urea, sulphated castor oil, polyvinylpyrrolidone, montmorillonite, CMC (carboxymethyl celluloses), sodiumalginate, xanthan gum, sodium silicate, acrylic acid copolymers, HMC(hydroxymethyl celluoses) HEC (hydroxyethyl celluloses) and mixturesthereof.

If a lubricity/calender aid is used in the coating or fillingcomposition of the present invention, it is generally present in levelsup to 2% by weight. Any art recognized lubricity or calendering aid maybe used. Appropriate lubricity or calendering aids include, but are notlimited to, calcium stearate, ammonium stearate, zinc stearate, waxemulsions, waxes, alkyl ketene dimer, glycols and mixtures thereof.

If an antifoamer or defoamer is used in the coating or fillingcomposition of the present invention, it is generally present in levelsup to 1% by weight. Any art recognized antifoamer or defoamer may beused. Appropriate antifoamer and defoamers include, but are not limitedto, blends of surfactants, tributyl phosphate, fatty polyoxyethyleneesters plus fatty alcohols, fatty acid soaps, silicone emulsions andother silicone containing compositions, waxes and inorganic particulatesin mineral oil, blends of emulsified hydrocarbons and other compoundssold commercially to carry out this function.

If dry or wet pick improvement additives are used in the coating orfilling composition of the present invention, they are generally presentin levels up to 2% by weight. Any art recognized dry or wet pickimprovement additives may be used. Appropriate dry or wet pickimprovement additives include, but are not limited to, melamine resin,polyethylene emulsions, urea formaldehyde, melamine formaldehyde,polyamide, calcium stearate, styrene maleic anhydride and mixturesthereof.

If dry or wet rub improvement and abrasion resistance additives are usedin the coating or filling composition of the present invention, they aregenerally present in levels up to 2% by weight. Any art recognized dryor wet rub improvement and abrasion resistance additives may be used.Appropriate dry or wet rub improvement and abrasion resistance additivesinclude, but are not limited to, glyoxal based resins, oxidizedpolyethylenes, melamine resins, urea formaldehyde, melamineformaldehyde, polyethylene wax, calcium stearate and mixtures thereof.

If a gloss-ink hold-out additive is used in the coating or fillingcomposition of the present invention, it is generally present in levelsup to 2% by weight. Any art recognized gloss-ink hold out additive maybe used. Appropriate gloss-ink hold out additives include, but are notlimited to, oxidized polyethylenes, polyethylene emulsions, waxes,casein, guar gum, CMC, HMC, calcium stearate, ammonium stearate, sodiumalginate and mixtures thereof.

If optical brightening agents (OBA) and fluorescent whitening agents(FWA) are used in the coating or filling composition of the presentinvention, they are generally present in levels up to 1% by weight. Anyart recognized optical brightening agents (OBA) and fluorescentwhitening agents (FWA) may be used. Appropriate optical brighteningagents (OBA) and fluorescent whitening agents (FWA) include, but are notlimited to, stilbene derivatives.

If a dye is used in the coating or filling composition of the presentinvention, it is generally present in levels up to 0.5% by weight. Anyart recognized dye may be used.

If a biocide/spoilage control agent is used in the coating or fillingcomposition of the present invention, it is generally present in levelsup to 1% by weight. Any art recognized biocide/spoilage agent may beused. Appropriate biocides/spoilage agents include, but are not limitedto, metaborate, sodium dodecylbenene sulphonate, thiocyanate,organosulphur, sodium benzonate and other compounds sold commerciallyfor this function such as the range of biocide polymers sold by CalgonCorporation.

If a leveling aid is used in the coating or filling composition of thepresent invention, it is generally present in levels up to 2% by weight.Any art recognized leveling aid may be used. Appropriate leveling aidsinclude, but are not limited to, non-ionic polyol, polyethyleneemulsions, fatty acid, esters and alcohol derivatives, calcium stearateand other compounds sold commercially for this function.

If a grease and oil resistance additive is used in the coating orfilling composition of the present invention, it is generally present inlevels up to 2% by weight. Any art recognized grease and oil resistanceadditive may be used. Appropriate grease and oil resistance additivesinclude, but are not limited to, oxidized polyethylenes, latex, SMA(styrene maleic anhydride), polyamide, waxes, alginate, protein, CMC,HMC and mixtures thereof.

If a water resistance additive is used in the coating or fillingcomposition of the present invention, it is generally present in levelsup to 2% by weight. Any art recognized water resistance additive may beused. Appropriate water resistance additives include, but are notlimited to, oxidized polyethylenes, ketone resin anionic latex,polyurethane, SMA, glyoxal, melamine resin, urea formaldehyde, melamineformaldehyde, polyamide, glyoxals, stearates and other materialscommercially available for this function.

If an insolubilizer is used in the coating or filling composition of thepresent invention, it is generally present in levels up to 2% by weight.Any art recognized insolubilizer may be used.

The substrate to be filled or coated may be selected from any artrecognized fibrous substrate. Substrates for use in the presentinvention include both wood based and woodfree substrates. Exemplarysubstrates according to the present invention are paper.

The present invention can be used in the production of all paper grades,from ultra lightweight coated paper to coated or filled board.

Lightweight coated, or LWC, paper is generally coated to a weight offrom about 5 g.m² to about 13 g.m² on each side, and the total weightper unit area of the coated paper is generally in the range of fromabout 49 g.m² to about 65 g.m².

LWC paper is generally used for printing magazines, catalogues andpromotional material. The coated paper is required to meet certainstandards of surface gloss and smoothness. For example, the paper isgenerally required to have a gloss value of at least about 32, and up toabout 60, TAPPI units, and a Parker Print Surf value in the range offrom about 0.5 to about 1.6 μm.

Ultra lightweight coated, or ULWC, paper is sometimes otherwise known aslight lightweight coated, or LLWC, paper and is used for catalogues andfor advertising and promotional material sent through the mail to reducemailing costs. The coating weight is generally in the range of fromabout 2 g.m⁻² to about 7 g.m⁻² on each side, and the total weight perunit area of the coated paper is generally in the range of from about 25g.m⁻² to about 48 g.m⁻².

Methods of coating paper and other sheet materials are widely publishedand well known. See, for example, Pulp and Paper International, May1994, page 18, et. seq. Sheets may be coated on the sheet formingmachine, i.e., “on-machine” or they may be coated on a coating machine,i.e., “off-machine.” Any art recognized coating method may be used toproduce the coated sheet according to the present invention. Accordingto one embodiment, the coating is applied to the sheet material by anapplicator, and a metering device assures that the proper level ofcoating composition is applied. When an excess of coating composition isapplied by the applicator, the metering device is downstream of theapplicator. Alternatively, the correct amount of coating composition maybe applied to the applicator by the metering device, such as a filmpress. While coating the sheet, the sheet may be supported by backingrolls or may be subjected only to tension.

Examples of known coaters which may be used with the present invention,include, but are not limited to, air knife coaters, blade coaters, rodcoaters, bar coaters, multi-head coaters, roll coaters, roll/bladecoaters, case coaters, liquid application systems, reverse roll coaters,extrusion coaters, and curtain coaters.

The present invention may be used in the production of coated paper forprinting. Any art recognized method of printing may be used with thecoated paper according to the present invention. Appropriate printingtechniques will be readily apparent to the skilled artisan. Printingtechniques for use according to the present invention include, but arenot limited to, rotogravure, offset printing, or flexo printing.Rotogravure involves the use of an engraved or etched cylinder as animage carrier. Image areas are etched or engraved below non-image areasin the form of tiny sunken cells. The cylinder is immersed in ink, andthe excess ink is scraped off by a blade. When the substrate contactsthe printing cylinder, ink transfers, forming the image. Offset printingis an indirect printing method in which the inked image on a press plateis first transferred to a rubber blanket that, in turn, “offsets” theinked impression to a press sheet.

The invention will be further clarified by the following examples, whichare intended to be purely exemplary of the invention.

EXAMPLES Example 1

This example illustrates a method of refining kaolin to produce a highshape factor kaolin, i.e., greater than or equal to 70, either as a drykaolin or a kaolin slurry that is stabilized by adding additional waterand carboxymethyl cellulose (a thickener). A “stabilized slurry” refersto a slurry where the kaolin does not appreciably settle out of solutionover time. It would be expected that adding additional water would haveprovided a less stabilized slurry. By including a CMC(carboxymethylcellulose) thickener, a stabilized slurry is formed whiledecreasing the slurry solids. The stabilization process is performed asfollows:

1. The product is reblunged with a small amount of dispersant, such as35/65 Mill Chem, which is a mixture of soda ash and polyacrylate.

2. Add approximately 8 lbs/ton CMC to thicken and stabilize.

3. Add biocide to prevent bacterial interaction with the CMC.

FIG. 1 is a flowchart outlining the process of providing a high shapefactor kaolin. A Georgia coarse crude kaolin is blunged, degritted, andsubjected to magnetic separation. After a first attrition grinding stepwith sand, the product is classified to separate the coarse fractionfrom the fines. The coarse fraction is subjected to a second attritiongrinding step, followed by leaching and filtering. A dry kaolin can beprepared by drying in a spray dryer, whereas a kaolin slurry can beprepared, such as a slurry further comprising CMC, biocide and adispersant.

PROCESS COARSE FRACTION FROM COARSE FRACTION FROM AFTER AFTER SPRAYPRODUCT 1^(ST) ATTRITION GRINDING 2^(ND) ATTRITION GRINDING LEACHING**DRY % solids 33.6% 33.1% 51.6% 1.0% s.g. 1.262 Tons 8.9 4.56 pH 7.3Brightness 86 L 94.27 a −0.2 b 2.6 TiO₂ 0.612 Fe2O₃ 0.44 Brookfield* 180Hercules* 450 Panacea 74.7 99.8 99.9 93 PSD   <2 m 39.3 56.9 56.5 56.0  <1 m 23.1 36.1 36.6  <.5 m 12.9 20.5 20.0 <.25 m 6.8 10.0 11.0Recovery 100.0% 99.0% 98.0% 99.0% *(60% solids) **2 lbs/ton sodiumhydrosulfite

Example 2

FIG. 2 is a flowchart outlining a process for the preparation of a highshape factor kaolin. A Georgia crude kaolin is subjected to blunging anddegritting steps, followed by magnetic separation and a first attritiongrinding step. The product is then classified where the coarse fractionis subjected to a second attrition grinding step, followed by anotherclassification to obtain a product where 70% of the particles have anesd less than 2 μm. This product is then leached and filtered, followedby the formation of either a dry kaolin product or a slurry.

COARSE FRACTION FROM PROCESS COARSE FRACTION FROM 2^(ND) ATTRITIONGRINDING AFTER AFTER SPRAY PRODUCT 1^(ST) ATTRITION GRINDING &CLASSIFYING LEACHING** DRY % solids 33.6% 25.5% 52.1% 1.2% s.g. 1.2621.184 Tons 8.9 3.58 Brightness 87.2 87.44 L 95.17 a −0.18 b 2.77 TiO₂0.613 Fe2O₃ 0.419 Brookfield* 160 Hercules* 480 Panacea 74.7 97.4 87.4PSD   <2 m 39.3 69.7 69.1   <1 m 23.1 48.0  <.5 m 12.9 28.4 <.25 m 6.815.0 Recovery 100.0% 57.3% 98.0% 99.0% *(60% solids) **2 lbs/ton sodiumhydrosulfite

Example 3

This Example provides another process where a Georgia crude kaolin isreblunged and degritted, as illustrated in the flowchart of FIG. 3.After selective flocculation, the kaolin is subjected to magneticseparation. A coarse fraction from a size classification centrifuge isthen added to the kaolin prior to a first attrition grinding step. Aftera size classification, the coarse fraction is subjected to a secondattrition grinding step, followed by leaching and filtering to produce ahigh shape factor product.

AFTER PROCESS Grinder Grinder Coarse LEACH- Spray Bird PRODUCT FeedProduct Fraction ING** Dry Rejects % solids 32.7 32.1 29.6 43.9 96.254.1 Brightness 84.56 85.83 85.98 87.67 87.21 82.36 L 94.6 95.02 95.0895.5 95.25 92.64 a −0.2 −0.18 −0.13 −0.22 −0.19 −0.27 b 4.19 3.83 3.83.11 3.11 3.11 TiO₂ 0.613 0.635 0.652 0.639 0.63 0.666 Fe2O₃ 0.394 0.4060.43 0.385 0.375 0.382 Brookfield* 1360 Hercules* 50 rpm Panacea 51.687.1 87.1 85.8 87.1 57.8 PSD <2 m 48 66.3 70.9 69.3 69.7 35.8 <1 m 27.244.2 49.2 46.9 47.4 20.4 <.5 m 13.6 24.3 28.3 26.3 26 10.1 <.25 m 4.87.8 10.6 10 11.6 2.7 Recovery 99% 88.1% 98% 99% 11.9% *(60% solids) **5lbs/ton sodium hydrosulfite

Example 4

FIG. 4 is a flowchart illustrating a process for making a barriercoating pigment from coarse fraction kaolin prepared according to themethod of Example 1, where the barrier coating comprises kaolin having ashape factor of at least about 90:1. 5 gallons of coarse hyperplatyfraction kaolin is obtained in accordance with one of the previouslydescribed embodiments of the invention. The slurry is adjusted to have asolids content of 35%. 1 lb/ton C-211 is added, and more can be added ifnecessary. The batch is subjected to sand grinding to increase thepercentage of particles having a size of less than 2 μm by 24% to 26%.3600 mL sand and 1800 mL clay slip was used. In the final slurry, atleast 55% of the solids had a size less than 2 μm.

The slurry was screened on a 325 mesh screen and flocculated to pH=3with sulfuric acid. After filtering, the filter cake was 35% SodiumPolyacrylate and 65% Soda Ash) to pH=6.5. Some of the reblunged filtercake was spray dried and added back to the slip to make a slurry havinga solids content of 58% to 60%, but the slurry could optionally be aslow as 50% solids or even 45% solids. The table below provides sizedistribution and shape factor values of the final product.

Brightness GE 85.79 PH 7.2 325 m Residue 0.0071 PSD % <10 μm 97.2 PSD %<5 μm 86.5 PSD % <2 μm 56.0 PSD % <1 μm 35.4 PSD % <.5 μm 18 PSD % <.25μm 9.7 PSD % <.1 μm 4 Visc. Brook #2 As Is 79 Visc. Herc rpm @ 18 dynes400 Visc: Solids % 59.2 Panacea (shape factor) 92

Kaolin samples A–C were tested for grease and oil barrier properties.Samples A and B are known kaolin products whereas sample C is a kaolinproduct prepared in accordance with the method of Example 1. Barriercoatings were prepared as described in this Example, with these kaolinproducts.

Sample A - Sample B - Sample C - Comparative Comparative Invention PSD %82.4 89.9 56.0 <2 μm Panacea- 35 58.9 92 Shape Factor

Barrier coatings were applied to a 60 g/m² woodfree base sheet with acylindrical laboratory coater. The coatings comprised the kaolin,calcium carbonate, latex, calcium stearate, and a viscosity modifier.The physical properties of the kaolins can be found in the table, above.The barrier coatings were applied twice on the same paper sample. Thecoated paper samples were finished on a laboratory scale calender.

The calendered paper samples were evaluated using a 3M Kit test inaccordance with TAPPI procedure T559 pm-96 and for oil and greaseresistance (OGR) in accordance with TAPPI procedure T509 cm-85. Whenusing these test methods, higher values using the 3M kit are superior,while lower values when testing for oil and grease resistance aresuperior.

The 3M test kit was developed to determine the effectiveness offluorochemical treated papers. The tests involve exposing the paper to aseries of solution, which contain castor oil, heptane and toluene. Thereare 12 solutions of levels in the Kit test. As the Kit levels increasethe proportion of solvent is larger.

The OGR test involves exposing the coated paper samples to a suitableoil or oil containing compound for a set period of time. The amount ofoil that seeps through the coated sample and is absorbed on anunderlying blotter is quantified. The test values from the OGR representthat percentage of the blotter paper covered by the oil. Thus, lowervalues are preferred. OGR values of 3 and less denote fully satisfactoryproducts.

3M Kit (Coated) OGR Sample No. Flat Creased Flat Creased Sample A >12 <39 17 Sample B >12 <3 11 21 Sample C >12 <3 3 5

As can be seen from the table above, the sample according to the presentinvention provides stronger barriers when compared with prior artkaolins. Such results can be attributed to the high shape factor andlarge plate diameter associated with the kaolins of the presentinvention.

Example 5

Example 5 illustrates the utility of the inventive kaolin compositionsas a component of a paper coating. Paper coating formulations were madeup based upon 100 parts of pigment with 14 parts Dow 692 latex binder, 4parts PROCOTE protein binder, 1.1 parts DISPEX polyacrylate dispersant,TiO₂ and kaolin were added as indicated in the Table, below.

Coating A B C D E F G Pigment Prior Art 70 — — — — — — Kaolin Invention— 70 85 100 — — — Kaolin 1 Invention — — — — 70 85 100 Kaolin 2 TiO₂ 3030 15 — 30 15 — Dispex 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Dow 692 14 14 14 1414 14 14 Procote 4 4 4 4 4 4 4 pH 8.5 8.5 8.5 8.5 8.5 8.5 8.5 BrookfieldViscosity Spindle #2 cps@10 156 96 128 244 60 100 384 rpm cps@20 108 68102 156 48 74 244 rpm cps@50 74 60 72 102 49 63 152 rpm cps@100 72 63 7096 53 70 154 rpm

The kaolin properties are set forth in the Table below:

Prior Art Kaolin Invention Kaolin 1 Invention Kaolin 2 Brightness 90 9192 2 micron 91 85.6 66.7 0.5 micron 35 34.4 18.1 Shape 15 107 141 FactorSurface 15 15.9 11.3 Area Steepness 45 37 33

The formulations were drawn down with wire wound rods onto a basestock.The fibrous substrate was coated to a coat weight of 15 gsm. Thecoatings were air dried and brightness and gloss were measured usingTappi standard methods. The results are set forth in the Table below.

Coating A B C D E F G Pigment 15.3 15.1 15.2 15.2 15.4 15.4 15.3 LevelBrightness ISO Mean 65.8 69.4 64.8 54.9 68.7 66.2 57.8 St. Dev. 1.151.16 1.06 0.86 0.55 0.72 1.46 St. Error 0.36 0.37 0.34 0.27 0.17 0.230.46 N 10 10 10 10 10 10 10 Sheet Gloss (75) Mean 13.0 9.4 10.9 11.2 9.59.1 10.1 St. Dev. 1.05 0.89 0.49 0.64 1.23 0.92 1.34 St. Error 0.27 0.230.13 0.17 0.32 0.24 0.35 N 15 15 15 15 15 15 15

Samples according to the present invention exhibited improved brightnessat accepted TiO₂ levels and comparable brightness at lower TiO₂ levels.See FIG. 5 which plots the brightness results noted above as a functionof TiO₂ content. Paper coated according to the present invention giveshigh brightness even with TiO₂, significantly above the brightness ofthe uncoated sheet.

Thus, the inventive kaolin compositions are useful in applicationswherein a high opacity, brightness or scattering are required, such asin brown board coating applications and may serve to reduce the amountof TiO₂ required therein. Further, although the present exampleillustrates the utility of the inventive kaolin as a paper coating, thecomposition could also be used to coat other substances wherein a highbrightness or opacity coating is needed.

Example 6

To test the utility of the inventive kaolins as a filler, handsheetswere prepared from a unbleached natural Kraft which has undergone norefining. While the present example pertains to use as a paper filler,it is anticipated that the inventive kaolins could also be useful as ahigh opacity filler for plastics and like materials.

The handsheets were made with a TAPPI sheet mold at a pH of 4.5. Thesheets were made on a fourdinier paper machine using unbleached hardwoodkraft. The conditions are discussed in the table, below.

pH 6.75 Freeness (CSF) 470 Freeness (CSF) after beating 380 Britt Fines% Fiber 80.3 % Fines 18.9 % Ash 0.8 Brightness 22.7 SpecificConductivity 1750 Mutek PCD: −340 Total Dissolved Solids  2327 ppmInorganic Dissolved  1880 ppm

The handsheets were air dried and a range of properties were measuredfor unfilled sheets, sheets made using the commercially available priorart pigment ASTRAPLATE (available from Imerys, Inc.) and two sets ofsamples using the composition identified as Invention Kaolin 2 inExample 5 above. The ASTRAPLATE and Invention 2 Samples included 2 lbsper ton of a commercially available retention aid of a sort that wouldbe readily obtainable by one of ordinary skill in the art.

Sample 1 2 3 4 5 Description Unfilled Unfilled Prior Art InventionInvention Astraplate Kaolin 2 Kaolin 2 % Ash (450)) 0.7 0.7 5.9 12.323.9 % Ash (900) 2.1 0.6 4.6 12.2 21.4 GSM 53.9 51.4 52.4 51.5 52.3Sheet Brightness 19.9 20.1 22.8 29.4 38.5 Opacity 88.4 88.2 90.3 94.296.2 F8 Sheet Scatter 179 220 236 307 524 F8 Absorption 271.5 318.8295.9 246.4 246.7 F10 Sheet Scatter 208 213 241 341 481 F10 Absortion176.1 168.7 168.3 157.6 132.7 Pigment Scatter — — 807 1073 1557 GurleyPorosity 13.22 11.23 13.97 20.25 22.25 Burst Factor 45.4 44.6 40.2 28.617.2 Sheffield Smooth- 293 277 276 261 252 ness

Example 7

Filled paper was made using a bleached hardwood, softwood mixture beatento a 400 Canadian Standard Freeness (CSF), a retention aid, andInvention Kaolin 2 of Example 5. The pH was maintained at 5.0. Handsheetwere made and allowed to air dry. A range of properties were measured.

Sample 1 2 3 4 Description Unfilled Invention Invention Invention Kaolin2 Kaolin 2 Kaolin 2 Retention Aid in Toms 1 1 1 1 Headbox % Ash (450)) —7.62 21.12 28.13 % Retention on (450) — 85 58.2 74.1 Handsheet % Ash(450) — 6.48 12.29 20.84 GSM 58.6 59.2 57.5 57.4 Sheet Brightness 83.885.3 85.9 86.7 Opacity 73.9 79.5 83.1 87.2 Sheet Scatter 351 457 541 695Absorption 4.9 5.8 6.3 7.1 Caliper 3.839 3.851 3.788 3.736 Bulk 1.661.65 1.67 1.65 Pigment Scatter — 1987 1897 2002 Gurley Porosity 7.9 10.29.3 14.2 Sheffield Smoothness 271293 251 244 206

As can be seen from the table above, the pigment light scatter accordingto the present invention (1900–2000 cm²/g) compares favorably with thatwhich can be achieved using calcined clay at 2500 and standard fillerclay at 1100.

Example 8

In this example, a series of coarse fractions, i.e., “b-fraction” clayswere subjected to attrition grinding to determine which clays result inhigh shape factor extenders while retaining particle size. Theproperties of the resulting materials were assessed to determine whichmaterials provided high opacity and low sheen in a high PVC paintformulation.

Carbolite grinding experiments with fine (20–40 lbs) Carbolite grindingmedia were performed on four materials: (1) Invention Kaolin 3, (seecolumn 0), having a coarse, platy character; (2) Invention Kaolin 4 froma Bird centrifuge, having a coarser, platy character (see column 0); (3)a Georgia feed clay (DBK 110 Contour 100, Imerys), a Dry Branch feedclay; and (4) a fine particle size clay (K20, Imerys), a Kaopaque 20product that had already been processed to a fine particle size.Physical properties of the ground clay are shown in the Table, below.

Physical properties of the ground clays. Energy Invention Kaolin 3Invention Kaolin 4 kwh/t 0 50 100 150 0 50 100 1560 Colour ISO B'ness81.5 84.7 85.7 86.5 74.7 75.7 75.4 75.7 Yellowness 6.9 5.8 5.6 4.8 8.38.7 9.1 8.9 L* 94.8 95.8 96.1 96.2 92.2 92.8 92.8 92.9 a* 0.24 −0.050.03 −0.13 0.77 0.62 4 0.54 b* 4.54 3.88 3.64 3.22 5.57 5.89 6.12 6.01Sedigraph <10 μm 92.5 98.9 98.7 99.5 87.6 99.2 98.7 99 <8 μm 86.9 97.898.6 98.8 72.6 98.5 98.3 98.4 <5 μm 86.6 91.5 96.7 97.1 64.5 96.3 96.396.5 <2 μm 27.5 62.9 79.1 83.1 28.3 76.7 81 83.7 <1 μm 18 41.8 59.6 64.819 53.3 62.6 67 <0.75 μm 14.9 33.8 50.8 57.3 16.3 44.8 55.4 59.5 <0.50μm 11.1 23.3 35.4 43 13.1 32.3 42.3 45.6 <0.25 μm 5.8 10.1 13.7 17.2 715.1 17.3 18.7 <0.10 μm 1.9 4.7 3.1 5.3 1.7 5.7 4.1 5.2 d50 2.94 1.220.77 0.64 3.01 0.86 0.67 0.6 Steepness* 32 30 32 31 27 30 29 31 S.F.^(†)28.8 89 96 98.8 18.7 101 103 98.3 S.A.^(‡) 7.49 13.22 9.52 13.48*Steepness is given by d30/d70* 100. Increasing steepness indicates anarrower PSD. ^(†)S.F. = Shape factor ^(‡)S.A. = Surface area

Energy Georgia Feed Clay Fine Particle Size Clay kwh/t 0 50 100 150 0 50100 150 Colour ISO B'ness 81.6 84.2 84.5 85 84.4 85.8 85.8 86.3Yellowness 7.9 6.7 6.5 6.1 6.4 5.6 5.7 5.3 L* 95.1 95.8 95.9 96 95.896.1 96.1 96.2 a* 0.16 0.02 0 −0.03 0.08 0.02 0.02 −0.03 b* 5.21 4.394.27 4 4.17 3.67 3.7 3.48 Sedigraph <10 μm 90.4 98.5 99.6 99.2 99.6 99.999.8 99.3 <8 μm 86.9 98.3 99 98.8 99.2 99.9 99.9 99.4 <5 μm 78.8 97.1 9899 95.5 99.2 99 99.3 <2 μm 55.6 84.4 87.8 93.4 79.7 91.4 92 95.2 <1 μm42.1 65.5 69 81.2 63.9 78.5 79.5 86.1 <0.75 μm 36.1 54.9 58.7 72.8 5772.1 72.1 80.4 <0.50 μm 27.4 39.7 43.1 56.3 45.7 59 58.4 68.2 <0.25 μm13.4 20.54 21.3 25.7 25 32.8 31.8 39.2 <0.10 μm 5.7 9 7.4 8.8 10.4 12.612.8 15.9 d50 1.4 0.65 0.59 0.44 0.6 0.4 0.41 0.32 Steepness 19 31 33 3825 31 32 35 S.F. 7.3 37.5 37.5 42 29.9 45.7 44.3 43.5 *Steepness isgiven by d30/d70* 100. Increasing steepness indicates a narrower PSD.^(†)S.F. = Shape factor ^(‡)S.A. = Surface areaOther Properties

Rheology

In all cases Carbolite grinding had a small but significant effect onthe rheology. As the grinding energy input increased there was anincrease in both the Brookfield and Rotothinner viscosities, while thehigh shear (cone and plate) viscosity did not change.

Scrub and Stain Resistance

As expected, scrub and stain resistances worsened as the opacityincreased.

Color

The ground Invention Kaolin 3 had good color, better than the existingfine particle size clay product. Further improvements may be possiblethrough bleaching of the ground clay.

Conclusions

The light Carbolite grinding of Invention Kaolin 3 can provide a productcomprising a coarse, high shape factor clay giving a combination of lowsheen with significant opacity improvements. These results reflect theperformance of a small batch of material. The more highly groundInvention Kaolin 3 can have a fine high shape factor clay giving largeopacity improvements, at the cost of increased sheen. Such a product mayoffer an alternative to calcined clays in formulations where high scrubresistance is not regarded as important.

Opacity and Sheen

FIG. 6 presents the opacity and sheen of the ground clays in the highPVC formulation. The clays were prepared by grinding with fine Carbolitemedia. The grinding energy is indicated by the size of each point. Toallow comparison some standard clays (SUPREME and GLOMAX LL*, all ofwhich are Imerys clay products) are plotted.

Invention Kaolin 3

The 50 kwh/t ground Invention Kaolin 3 rejects gave a noteworthycombination of high opacity and low sheen.

The 100 kwh/t ground Invention Kaolin 3 gave high opacity with highsheen.

Other Feeds

Invention Kaolin 4 had a poor color, a substantial part of their opacityoriginated from absorption, rather than scattering of light. They alsoground to a finer particle size than Invention Kaolin 3, resulting inhigher sheen.

A 1.5 unit increase in opacity was achievable by Carbolite grinding ofthe fine particle size clay feed material. Similar opacities wereobtained with the ground Georgia Feed Clay. For these materials, anyincrease in opacity was always accompanied by a significant increase ofthe sheen, brought about by a decreasing particle size.

Chemical and mineralogical analysis of the US feed clays are provided inthe Table below.

Chemical and mineralogical analysis of the clays used in Example 8.Invention Invention Georgia Feed Fine particle Kaolin 3 Kaolin 4 Claysize clay XRF Analysis SiO₂ 45.08 44.93 46.51 46.49 Al₂O₃ 39.54 38.9137.79 37.91 Fe₂O₃ 0.29 0.61 0.33 0.44 TiO₂ 0.89 1.42 1.24 1.05 CaO 0.050.01 0.05 0.05 MgO <.01 0.02 0.04 0.05 K₂O 0.23 0.56 0.04 0.05 Na₂O 0.140.21 0.09 0.09 L.O.I. 13.79 13.33 13.91 13.88 XRD Analysis Kaolin 98 9499 98 Mica 2 6 1 2 Quartz <0.5 <0.5 0 0 Feldspar 0 0 0 0

FIG. 7 is a plot of the d₅₀ versus shape factor. For each of the fourfeeds, the Carbolite grinding resulted in an increase in the clay'sshape factor, and a decrease in the mean particle size. From FIG. 7, itcan be seen that Invention Kaolin 3 and Invention Kaolin 4 yielded thehighest shape factors. Invention Kaolin 4 suffered from poor color, withan ISO brightness of 75.7.

Example 9

The ground products of Example 8 were tested in a series of paints usinga generic high PVC paint formulation, as exemplified in the Table below.

Simplified matt paint formulation. Weight/pounds Weight % per US gallonTipure R706 TiO₂ 9.0 1.11 Extender 10.0 1.23 Carbital 120 30.0 3.70Dispex N40 0.35 0.04 Calgon S 0.05 0.01 Ammonia 0.880 0.15 0.02 Natrosol250 MBR 0.3 0.04 Nopco NXZ 0.3 0.04 Proxel GXL 0.1 0.01 Propylene Glycol2.0 0.25 Texanol 1.4 0.17 Acronal 290D latex 13.0 1.60 Primal TT935 0.80.10 Water 32.55 4.02 Total = 100.00 % PVC 74.09 Specific gravity, g/cm³ 1.479 Solids wt. % 56.25 Solids vol. % 34.35

The opacity, gloss, stain resistance, BS scrub loss and mud crackresistance values are shown in the Table below.

Comparison of the Carbolite ground US clays as extenders. 159 Paint No.→ 154 G.F. 160 161 162 163 Extender Supreme C.** G.F.C G.F.C I.K.4*I.K.4 Work/kwh/t — 50 100 150 50 100 Brookfield viscosity, poise @1r.p.m. 2960 1250 1340 1580 1160 1270 @10 r.p.m. 430 202 214 239 182 213@100 r.p.m. 65.4 42.2 42.5 44.7 40 43.8 Rotothinner, 12.1 9.7 9.9 1010.4 10.6 poise Cone & 1.4 1.2 1.2 1.2 1.3 1.3 Plate, poise Opacity @90.3 90.1 90.2 90.8 92.7 93.0 20 m²/l S, mm⁻¹ 78.2 76.1 76.6 80.3 90.192.0 K, mm⁻¹ 0.3 0.4 0.4 0.4 0.7 0.7 Gloss @ 60° 3 3.3 3.3 3.2 2.9 2.9Gloss @ 85° 3.4 3.3 3.7 3.7 3.7 3.9 L* 96.39 96.22 96.25 96.28 95.3995.34 a* −0.68 −0.39 −0.38 −0.4 0.03 0.05 b* 2.21 3.38 3.37 3.24 3.78 4Stain 83.4 80.6 79.2 80.0 72.9 73.4 resistance, % BS scrub 5.80 6.246.26 6.34 8.14 8.17 loss^(†), mg cm⁻² Mud crack 1500 1350 1350 1500 15001400 resistance, μm *I.K.4 = Invention Kaolin 4 **G.F.C = Georgia FeedClay ^(†)BS scrub loss, 100 cycles, 7 days

155 156 157 158 Paint No. → 164 165 F.P. F.P. F.P. F.P. ExtenderI.K.3^(‡) I.K.3 S.C.* S.C. S.C. S.C. Work/kwh/t 50 100 0 50 100 150Brookfield viscosity, poise @1 r.p.m. 1220 1370 1190 1470 1520 1730 @10r.p.m. 189 210 209 249 246 277 @100 r.p.m. 42 43.5 44.6 45.8 46 50.5Rotothinner, 10.2 10.3 10 10.4 10.4 10.6 poise Cone & Plate, 1.2 1.3 1.31.3 1.2 1.3 poise Opacity @ 20 90.9 92.0 89.3 90.8 90.6 90.9 m²/l S,mm⁻¹ 81.9 89.6 71.7 80.0 79.0 81.1 K, mm⁻¹ 0.3 0.3 0.4 0.4 0.4 0.4 Gloss@ 60° 3.3 3.3 3.2 3 3.3 3.1 Gloss @ 85° 2.9 3.8 2.8 3.6 3.5 3.5 L* 96.7196.86 96.19 96.32 96.32 96.4 a* −0.43 −0.44 −0.41 −0.42 −0.42 −0.37 b*2.62 2.48 3.25 3.03 3.05 2.96 Stain 73.3 72.3 82.5 80.1 81.9 82.5resistance, % BS scrub loss†, 7.19 7.96 5.66 6.67 6.58 7.02 mg cm⁻² Mudcrack 1350 1500 1350 1050 1500 1150 resistance, μm ^(‡)I.K.3 = InventionKaolin 3 *F.P.S.C. = fine particle size clay

In all cases, Carbolite grinding had a small but significant effect onthe rheology. As the grinding energy input increased, there was anincrease in both the Brookfield and Rotothinner viscosities, while thehigh shear (cone and plate) viscosity did not change.

Scrub and stain resistances worsened as the opacity increased.

Invention Kaolin 3 had good color in that it is better than that of theexisting fine particle size clay product.

Example 10

FIG. 8 is a flowchart outlining a process where two coarse fractions arecombined, followed by leaching and filtering to produce a stabilizedhigh-solids slurry by adding a stabilizing agent such ascarboxymethlycellulose or a smecite clay (i.e. bentonite, hectorite,montmorillonite, etc.). To illustrate this, a two tote sized samples(˜250–300 gallons) of the inventive product of Example 9 were made downby blunging with a 65/35 SAPA (soda ash/sodium polyacrylate) dispersantpackage to a pH of 6.5. The two sample totes of the product were eachtreated with biocide (1 lb/dry tonne Proxel) then treated with 8 lbs/drytonne of the carboxymethylcellulose PE-30 EX. The final solids contentof Sample 1 was 52% and the final solids content of sample 2 was 49.6%.The initial Hercules viscosity of samples 1 and 2 were measured using ano. 2 spindle at 20 rpm and found to be 30 cps and 18 cps respectively.The initial Hercules viscosity of sample 1 was measured as 8.5 dynes at4400 rpm at pH 7.2 and the initial Hercules viscosity of sample 2 wasmeasured as 5.0 dynes at pH 7.0.

Next, the stability over time of the slurries was measured and theresults are displayed in the following table. One pint samples were setaside and allowed to settle for a desired amount of time, after whichthe Brookfield viscosity of the top and bottom portion of each samplewas measured using a ‘T’ bar at ½ rpm. If the slurry was not stable, onewould expect the measured viscosity to rise over time to a maximum valueof approximately 400,000.

Sample Loc. 0 Day 14 Day 21 Day 28 Day Sample 1 - Top 6,800 16,00015,200 14,000 Sample 1 - Bottom 6,800 18,000 22,400 21,200 Sample 2 -Top 6,000 14,400 14,400 14,000 Sample 2 - Bottom 6,000 14,800 16,40017,600

At the end of the 28 day test period, each one pint sample was subjectedto a pour test wherein it was upended and allowed to pour for 1 minute.In this time approximately 93.4% sample 1 and 94.9% of sample 2 was seento pour from the container, illustrating the continued fluidity of thesample. A similar test performed on each entire tote (˜250–300 gallons)of samples 1 and 2 resulted in 99.7% and 99.2% pouring respectively. Ascan be seen from the pour test results and the above table, the CMCstabilized slurry is basically stable, which is surprising in light ofthe fact that the make down process with CMC results in an effectivedecrease in the slurry solids. Generally this process should be usefulin relation to slurries having a solids content, for example, in therange of greater than about 45%, between about 45% and about 60%, oreven in the range of about 50 to about 60%.

Unless otherwise indicated, all numbers expressing quantities ofingredients, reaction conditions, and so forth used in the specificationand claims are to be understood as being modified in all instances bythe term “about.” Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the following specification andattached claims are approximations that may vary depending upon thedesired properties sought to be obtained by the present invention.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification. Other embodiments ofthe invention will be apparent to those skilled in the art fromconsideration of the specification and practice of the inventiondisclosed herein. It is intended that the specification and examples beconsidered as exemplary only, with a true scope and spirit of theinvention being indicated by the following claims.

1. A method of refining kaolin, comprising: providing a kaolin slurry;attrition grinding the kaolin slurry; removing a coarse particle sizefraction from the slurry by centrifugation; and attrition grinding thecoarse-particle size fraction to provide a kaolin product having a shapefactor of at least about 70:1.
 2. The method according to claim 1,wherein the kaolin slurry comprises whole crude kaolin or a blend ofwhole crude and coarse-particle size fractions from a centrifuge.
 3. Themethod according to claim 1, wherein the kaolin product has a shapefactor ranging from about 70:1 to about 140:1.
 4. The method accordingto claim 1, wherein the kaolin product has a shape factor ranging fromabout 80:1 to about 100:1.
 5. The method according to claim 1, whereinthe kaolin slurry is beneficiated prior to the attrition grinding. 6.The method according to claim 1, wherein the kaolin slurry is degrittedprior to the attrition grinding.
 7. The method according to claim 1,wherein the kaolin slurry comprises a blunged-degritted-brightnessbeneficiated slurry.
 8. The method according to claim 1, wherein thecoarse particle size fraction that is removed comprises kaolin particlescomprising from 80% to 95% by weight of the particles having an esd ofless than 2 μm.
 9. The method according to claim 1, wherein the kaolinproduct resulting from attrition grinding the coarse particle sizefraction comprises particles having an esd ranging from 0.2 μm to 1.7μm.
 10. The method according to claim 1, wherein the kaolin productresulting from attrition grinding the coarse particle size fractioncomprises particles having a shape factor value ranging from 80:1 to110:1.
 11. The method according to claim 1, wherein the kaolin producthas a shape factor of at least about 80:1.
 12. The method according toclaim 1, wherein the kaolin product has a shape factor of at least about90:1.
 13. The method according to claim 1, wherein the kaolin producthas a shape factor of at least about 100:1.